1. Field of the Invention
The present invention relates to an electrophotographic type of image forming apparatus such as a printer and a copying machine.
2. Related Background Art
A jumping development method has been conventionally known, which uses a one-component developer for development. In the developing process, the one-component developer is made to fly up from a developer carrier to a required minute gap between the developer carrier and an image holding member facing each other. Such a method is disclosed in Japanese Patent Post-Exam Publication No. 41-9476. Another development technique is also known, which is to apply a high-frequency pulse bias (a frequency of 10 to 300 kHz) to the above-mentioned gap so that the developer will adhere to an image portion on the image holding member but not to a non-image portion (for example, refer to U.S. Pat. No. 3,866,574).
Still another development technique is known, which uses a two-component developer containing toner and carrier for development. In the developing process, the two-component is indiscriminately put into contact with both the image and nonimage portions on the image holding member while applying a low-frequency alternating electric field between the developer carrier and the image holding member, so that the developer will adhere substantially to the image portion along without adhesion to the nonimage portion (for example, refer to Japanese Patent Application Laid-Open No. 55-32060).
FIG. 4 schematically shows an example of an image forming apparatus provided with a conventional developing apparatus. In the developing apparatus of this example, a developing sleeve 120 as the developer carrier and an electrophotographic type of a drum-shaped photosensitive member or a photosensitive drum 1 as the image holding member are opposed to each other with a gap or developing area 113 therebetween so that an alternating electric field will be applied to the developing area 113.
To be more precise, the developing apparatus 110 is arranged to face the photosensitive drum 1 that rotates in the direction indicated by the arrow. In operation, well-known electrostatic latent image forming means 102 including a charger and exposing means form an electrostatic latent image on the photosensitive drum 1. The exposing means may be projection means for projecting an optical image of an original or an optical system that scans a laser beam modulated by a recorded image signal.
The developing apparatus 110 develops the electrostatic latent image formed on the photosensitive drum 1 to for a toner image. Well-known transferring means 103, including a transferring charger, transfer the toner image obtained to a transferring material such as paper. The transferring material with the toner image thereon is then separated from the photosensitive drum 1 and forwarded to fixing means 105 in which the toner image is fixed on the transferring material.
After the completion of the transferring process, toner particles remaining on the photosensitive drum 1 are removed by cleaning means 104 including a cleaning blade.
The developing apparatus 110 houses a nonconductive one-component developer 111 in a developing container 112. The one-component developer, that is, the toner 111 is carried by the developing sleeve 120 from the developing container 112 to the developing area 113 opposed to the photosensitive drum 1. The photosensitive drum 1 and the developing sleeve 120 are opposed to each other through the developing area 113 with a minute gap of 50 to 500 xcexcm therebetween. In the developing area 113, the toner 111 is applied to the latent image on the photosensitive drum 1 to develop the same.
The toner 111 on the developing sleeve 120 carried to the developing area 113 is regulated by a blade 116 as a regulating member to form a toner layer 111a thinner in thickness than the gap between the blade 116 and the sleeve 120.
As discussed above, the developing apparatus 110 shown in FIG. 4 performs so-called non-contact development. In other words, since the toner layer 111a formed by the toner carried to the developing area 113 is thinner in thickness than the minute gap between the developing sleeve 120 and the photosensitive drum 1, the toner 111 flies up from the developing sleeve 120 through the air gap to the photosensitive drum 1. To improve developing efficiency in the developing process and hence form a high-density, vivid developed image with less fogging, a bias power supply 118 for constant-potential control applies a developing bias voltage containing an alternating component to the developing sleeve 120.
Such a developing bias makes it possible to alternately actuate an electric field provided in such a direction as to transfer the toner 111 from the developing sleeve 120 to the photosensitive drum 1 and an electric field provided in such a direction as to reversely transfer the toner 111 from the photosensitive drum 1 to the sleeve 120, resulting in an excellent developed image.
A magnetic pole S1 of a magnet roller 115 housed in the developing sleeve 120 forms a magnetic field in the developing area 113 to prevent fogging, making a line image vivid.
In the above-mentioned conventional image forming apparatus, however, the photosensitive drum 1 and the developing sleeve 120 rotate at a constant peripheral speed, for example, of 24 mm/s, regardless of whether the image to be developed has a relatively low image ratio such as a text or a relatively high image ratio. As a result, both gain an identical throughput, for example, of 4 ppm. Such an image forming apparatus may be inferior to another type, for example, an ink jet type, of image forming apparatus which varies throughput to an image having a relatively low image ratio such as characters may not be as good as that gained by another type of image forming apparatus.
It is an object of the present invention to provide an image forming apparatus capable of improving throughput, particularly to an image having a low image ratio.
It is another object of the present invention to provide an image forming apparatus capable of preventing deterioration of developer caused when an image is developed at a high image ratio, and preventing the life of a developing apparatus from being shortened due to the deterioration of the developer.
It is still another object of the present invention to provide an image forming apparatus comprising an image holding member for holding an electrostatic latent image, and a developer carrier for carrying developer to a developing portion, wherein a speed ratio between the image holding member and the developer carrier vary according to an image ratio.
It is yet another object of the present invention to provide an image forming apparatus comprising an image holding member for holding an electrostatic latent image, a developer carrier for carrying developer to a developing portion, and other developer carriers for respectively carrying developer of other colors to the developing portion, wherein a speed ratio between the image holding member and each of the developer carriers vary according to an image ratio for each color.
The above and other objects and features of the present invention will become more apparent by reading the following detailed description in connection with the accompanying drawings.